Instruments for radiometrically determining temperatures are calibrated against blackbodies. The characteristics of blackbody cavities, especially their temperature uniformity and effective emissivity, are important in establishing traceability to the International Practical Temperature Scale. When used with non-black surfaces, the radiation thermometer indicates an apparent temperature. The user must recognize how to utilize information such as target emissivity and temperature of the surroundings to properly infer true surface temperature from the indicated value.